DESCRIPTION: Phosphorylation of the c-terminus of the c-src protein at tyrosine-527 suppress its tyrosine kinase activity and transforming potential. The c-termini of other non-receptor tyrosine kinases encoded by the c-fyn, c-lck, and c-fgr proto-oncogenes are also phosphorylated at similar sites. The mechanisms by which c-terminal phosphorylation may regulate the activities of nonreceptor tyrosine kinases are not known. Recent work in the principal investigator's lab has demonstrated that a synthetic phosphotyrosine-containing peptide modeled on the c-terminus of the c-src protein binds to a highly transforming activated mutant of the c-src protein. Only traces of the normal c-src protein are bound to the phosphorylated peptide. Additionally, the phosphopeptide inhibits the kinase activity of the activated c-src and v-src proteins. The long term objective of the proposed studies is to understand how c-terminal phosphorylation regulates the c-src protein. The specific aims of the proposed research are: (1) To identify the binding sites on the activated c-src protein for the phosphorylated c-terminal peptide; and (2) To use a series of overlapping peptide analogs to identify the structural features of the c-terminal phosphopeptide that are important for binding and inhibitory activity. These studies will shed light on the mechanisms by which c-terminal phosphorylation may regulate src family kinases. Identification of the structural features of the phosphorylated peptide that are required for binding and inhibitory activity may lead to the rational design of compounds that modulate the kinase activity and perhaps the transforming potential of activated c-src mutants.